1. Field of the Invention
The present invention relates to connection of electrical generation equipment to an electrical grid, and in particular, the invention relates to connecting a wind turbine to an electrical grid as a part of a start-up procedure.
2. Description of the Related Art
The stability of electrical distribution grids is of increasing interest. For example, with ongoing de-regulation of electrical power generation and with an increasing share of electrical power generating equipment using renewable sources (such as wind), stability margins in the grid have become increasingly smaller. The reduced margin has led to more frequent grid outages. At least two issues are of considerable interest when considering aspects of grid start-up and making electrical connections.
First, recovery from grid outages requires at least one and perhaps several generating units to be started without the presence of a grid signal (i.e., an active grid). This requirement for “black starting” is imposed upon many types of generators, including wind turbines. Consider that with the increased use of wind turbines as well as increased power output, wind turbines are greater source of electrical power than ever before. Therefore, one skilled in the art will recognize that wind turbines with black start capabilities could be strategically located inside grid segments that are subject to an outage. The black start capable wind turbines may then be relied upon for energizing each grid segment.
In this scenario, it may be considered that each wind turbine would then form an “island” within a larger grid. Island grid operation may be temporary, as during the recovery from a larger grid outage, or permanent as in the case of autonomous grids. Unfortunately, black start capability is a new feature for wind turbines. No solutions or products are known to the inventors of the teachings herein.
Further, many wind turbines are coupled to electric grids by use of long cables. The long cables usually have a large electrical capacitance. Connection of wind turbines having long cables can lead to instantaneous transfer of undesirably high charge currents as well as transient over voltages, particularly if the wind turbine is connected at full voltage.
What are needed are techniques for controlled and safe start-up of wind turbines. Preferably, the techniques provide for energizing connections having large capacitance (as in the case of long cables), as well initiation of grid operation.